1. Field of the Invention
The present invention relates to an automatic address assignment apparatus, a control method therefor, and a program therefor. More particularly, the present invention relates to an automatic address assignment apparatus for assigning an IP address to a DHCP (Dynamic Host Configuration Protocol) client connected to a network, a control method therefor, and a program therefor.
2. Description of the Related Art
In general, all terminals connected to a network compatible with TCP/IP (Transmission Control Protocol/Internet Protocol) are distinguished by specific identification information called an IP address. The operation for setting such an IP address is troublesome for users who do not have knowledge of the IP, and, also, mistakes of setting a duplicate IP address cannot be prevented.
Accordingly, an automatic address assignment apparatus (what is commonly called a DHCP server) is connected to a network, and an IP address is automatically assigned by the DHCP.
Terminals receiving an assignment of an IP address from the DHCP server are called DHCP clients. First, the DHCP client broadcasts a “DHCP discover message” on the network. The DHCP discover message contains the media access control (MAC) address of the DHCP client, for example, specific identification information that is given to, for example, an Ethernet (registered trademark) card.
When the DHCP server receives a DHCP discover message, the DHCP server broadcasts on the network a “DHCP offer message” in which the IP address of itself (the IP address of the DHCP server) is set at the address of the transmission source. Since this DHCP discover message contains the MAC address of the transmission source (the DHCP client) of the DHCP discover message, the DHCP client can recognize that it is a DHCP offer message destined to itself.
The DHCP client collects DHCP offer messages for a fixed time, and when one DHCP offer message is received during that collection period, the DHCP client transmits a REQUEST message to the DHCP server which has transmitted the DHCP offer message. Alternatively, when a plurality of DHCP offer messages are received, the DHCP client selects one of the messages, and transmits a REQUEST message to the DHCP server which has transmitted that DHCP offer message. The REQUEST message contains information about which IP address is desired to be used. The DHCP server receiving the REQUEST message returns an acknowledgement (ACK) message if the IP address to be assigned is not yet used.
In the manner described above, in response to a request from each terminal (DHCP client) in the network, the DHCP server searches for an unused IP address from the range of IP addresses managed by itself and lends (also called leases) the address to a DHCP client (terminal) of the requesting source. The DHCP server is indispensable for a small-scale office or a home network, in particular, where a specialized network administrator cannot be assigned.
However, a conventional, automatic address assignment device leases an IP address to a requesting source in response to a request from a DHCP client. For example, when a plurality of DHCP servers exist on the network, contention of the DHCP servers is caused to occur, and confusion is brought about on the network. In such a case, in the network where there is a specialized administrator or a user who is familiar with IP technology, contention can be avoided by stopping unnecessary DHCP servers or by making the range in which the respective DHCP servers (sub-net masks) are in charge different. However, in a small-scale office or in a home network where there is not such an administrator or user, solution of a trouble is not easy, and the network cannot be used at all until a recovery from the trouble is made.